Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For ion concentration
Patent
1998-02-04
2000-07-18
Tung, T.
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For ion concentration
2057945, 204416, G01N 2726
Patent
active
060902698
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Speciation is one of the major challenges present in analytical chemistry.sup.1. Of particular interest is metal speciation. For example, while Cr(III) is essential to our bodies and part of our daily diet, Cr(VI), i.e., chromate, is highly toxic to human causing gastrointestinal disorders, dermatitis, uncertain of skin and is a suspected carcinogenic agent. Thus, the determination of trace levels of Cr(VI), that are often below 1 ppb in natural waters and in biological fluids in the presence of relatively high concentrations of Cr(III) is of particular importance. Although the redox speciation of chromium has been accomplished by separate preconcentration of Cr(III) and Cr(VI) fractions using chelating resins, coprecipitation, ion chromatography and solent extraction, such procedures are obviously complicated.sup.2. Electro-analytical methods that are potentially sensitive for redox-specification have also been used to determine chromate.sup.3. Although Cr(VI) has been preconcentrated and determined on mercury and chemically modified solid electrodes, most of these interfaces exhibit moderate stability and selectivity when employed in natural samples.
SUMMARY OF THE INVENTION
According to the present invention there is provided an assay of extremely high sensitivity for the selective determination of Chromium (VI). There is also provided a selective electrode for such determinations based on the molecular design of the solid-liquid interface, using a self-assembled mono-layer. By structuring the solid liquid interface using a self-assembled monolayer (SAM), a highly sensitive electrode exhibiting speciation capabilities toward Cr(VI), was developed. Application of SAMs in electroanalytical chemistry presents a very attractive approach as a means of assembling selective electrodes. SAMs offer highly organised systems in which the solid-liquid interface can be pre-designed at the molecular level in order to acquire a modified surface with desired properties. For example, Rubinstein and co-workers.sup.4 demonstrated that a mixed functionalised SAM recognised selectively Cu.sup.2+ ions in the presence of other ions. More recently, the inventor presented a highly sensitive electrode for cadmium ions using .omega.-mercapto-carboxylic acid monolayers.sup.5.
Thus, according to one aspect, the invention relates to a highly sensitive electrode for the selective quantitative detection and determination of Cr(VI) which comprises a gold electrode provided with a self-assembled monolayer (SAM) of a compound of the formula ##STR1## where n is zero or an integer from 1 to 18, preferred are electrodes where the SAM is one of positively charged 4-(2-ethanethiol) pyridinium. The invention also relates to a process for the preparation of Cr(VI)-selective gold electrodes which comprises polishing a gold surface of an electrode and contacting it with a dilute solution of a compound defined above, in the presence a mineral acid. A preferred mineral acid is sulfuric acid, and it is preferred the use of a solution of the pyridinium compound of about 5 mM and that of the sulfuric acid of about 0.1 M. Another aspect is a highly sensitive selective assay for Cr(VI), in the presence of Cr(III) and other cations, which is carried out by voltammetry using an electrode as defined above. Preferably there is used cyclic or square wave voltammetry. The assay is suitable for determinations in the range as low as the order of 1 ppt (part per trillion, 10.sup.12). There may be prepared a calibration curve of concentrations in the 1 ppt range is prepared, and actual results are read with the aid of such calibration curve. The can be carried out in the presence of Cr(III), Cu.sup.2+, Ag.sup.+, and Fe.sup.3+, of a concentration greater than up to about 10.sup.7 as large than the Cr(VI) concentration, which ions do not interfere with the accuracy of the assay.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated in the following with reference to the enclosed Figures, in which:
FIG. 1: A--Square wave v
REFERENCES:
Xie et al. "Binding of cytochrome c with 4-pyridyl derivatives modified on old electrodes", Electroanalysis (N.Y.) 1994, 6(7), 567-70), month unknown.
Andreu et al. ("Discreteness-of-Charge Effects at Molecular Films Containing Acid/Base groups", J. Phys. Chem, 1994, 98(48), 12753-8), month unknown.
Ure, A.M. et al., Ed., "Chemical Speciation in the Environment," Blackie Academic & Professional, pp. 363-365, (1995), month unknown.
Naghmush, A.M., et al., "Determination of Chromium in Different Oxidation States by Selective On-Line Preconcentration on Cellulose Sorbents and Flow-Injection Flame Atomic Absorption Spectrometry," Analytica Chimica Acta, vol. 288, pp. 247-257 (1994), month unknown.
Bauer, L., et al., "Addition of Thiourea to 2- and 4- Vinylpyridines, " Journal of Organic Chemistry, vol. 26, pp. 82-85 (1961), month unknown.
Gui, J.Y., et al., "Surface Chemistry of Mercaptopyridines at Ag(111) Electrodes Studied by EELS, LEED, Auger Spectroscopy and Electrochemistry," J. Electronanal. Chem., vol. 292, pp. 245-262 (1990), month unknown.
Cox, J.A., et al., "Stripping Voltammetry of Chromium (VI) at a Poly(4-Vinylpyridine)-Coated Platinum Electrode," Analytica Chimica Acta, vol. 154, pp. 71-78 (1993), month unknown.
Elleouet, C., et al., "Determination of Trace Amounts of Chromium (VI) in Water by Electrochemical Methods," Analytica Chimica Acta, vol. 257, pp. 301-308 (1992), month unknown.
Mandler Daniel
Turyan Iva
Noguerola Alex
Tung T.
Yissum Research Development Company of the Hebrew University of
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